1. Field of the Invention
The present invention relates to an image reading apparatus providing a line image sensor of contact type for line scanning a document.
2. Description of the Prior Art
The color CCD (charge coupled device) image sensor of contact type is widely used for the color image reading apparatus. An image of a document is dissolved into plural light components (usually three primary color red (R), green (G) and blue (B) components) and those light components are transduced to color data by photo-electric transducers, respectively. Then, respective color data is converted into digital data. These digital data are binarized according to a predetermined method such as the dither method after shading correction to them is performed. The binarized data are outputted to an external device such as a printer.
The CCD sensor of contact type has a limitation with respect to the scanning speed thereof and, accordingly, it is impossible to read a document faster than the limitation. A time interval necessary for reading a document is increased as the integration density of the CCD image sensor is increased.
In a highly integrated CCD image sensor, plural CCD sensor chips are arranged alternatively along the lengthwise direction of the CCD image sensor and each chip provides a signal output terminal. In a conventional CCD image sensor, respective chips are driven serially in the order of alignment of them and respective output signals from them are transmitted, via analogue switches, to a single signal line.
According to this conventional signal processing, it is difficult to speed up the reading speed of a document image.